A320 Captain reported a fumes event during pre-flight after APU bleeds were activated.

Narrative

Arrived at aircraft to find Maintenance dealing with a write up from the inbound flight. When the APU started and bleed turned on an inappropriate odor became noticed immediately. The crew exited the aircraft and we turned off the bleed and APU; then referenced the FOM. was already talking with Maintenance Control and made the logbook write up. Maintenance needed an engine run for troubleshooting the previous write up (this run was also used to troubleshoot the odor) so we did an external air start (believing the APU was causing the odor/fumes). After the start was complete and packs turned on; the odor/fumes again emanated from the gasper vents. After shut down the First Officer and I left the aircraft and I contact Dispatch and conferenced with Maintenance Control. The odor/fumes were intense enough to only abate when the aircraft doors were opened for ventilation. We both had slight headaches for a couple of hours after our last exposure. I had a feeling of light headed; or fog for a couple hours after as well. The flight was canceled and we went to rest. As of this writing I do not have any symptoms presenting that I believe stem from this event.